chapter 2 porosity

Porosity

•  Theory on porosity pores in rocks, pore types, porosity!

•  Determination of porosity!Determination of porosity in laboratory!

Porosity Theory on porosity

Porosity

Theory on porosity

Reservoir rock has interconnected pores filled with fluid (oil, gas, water) that can flow through the rock. There are also pores that are not interconnected (isolated pores).!

Porosity is the fraction of void space in a rock to its bulk volume .!

The unit for porosity in the Eq 1 is in percent. The unit can also be in fraction.!

Eq 1!

rock of ebulk volum

space pore of volume

100

=

=

∗=

b

p

b

p

VV

VV

φ

Generally;!

Porosity

Theory on porosity

Porosity

Porosity can be classified into;!1. Original porosity!2. Induced porosity!

The original porosity is developed during the deposition of the rock material.!

!intergrannular porosity in sandstone !!intercrystalline and oolitic porosity in limestones!

Induced porosity is developed by geologic process subsequent to the deposition of the rock.!

!fractures !!vug or solution cavities commonly found in limestones !

Theory on porosity!

Porosity

Sand grain

Effective porosity/interconnecte (25%)

Cementing material

Isolated porosity (5%)

Total porosity

(30%)

Theory on porosity!

Porosity

3 types of porosity!1. Effective porosity!2. Isolated porosity!3. Total porosity!

Effective porosity only consider interconnected pores.!

Isolated porosity refers to the isolated pores.!

Total porosity includes both interconnected and isolated pores.!

Which type of porosity is important in reservoir engineering?!

Theory on porosity!

Porosity

3 types of porosity!Theory on porosity!

Porosity

Theory on porosity!

Porosity

Porosity is influenced especially by:!1. Arrangement and shape of the grains!2. Mixture of various shapes and sizes of the grains!3. Amount of cementing material!

Large sphere!

Small sphere!Cubic packing!

48%!Rhombohedral

packing!26%!

Varied sizes of spheres affect the type and volume of the

pore.! Effect of cementing material!

Porosity 36%!

Porosity 20%!

Theory on porosity!

Porosity

Porosity measurement!

ebulk volum

pore of volume

100

=

=

∗=

b

p

b

p

VV

VV

φ

Measurement made in lab:!Vp , volume of pore can be directly or indirectly measured!

Vb , bulk volume can be measured directly or indirectly!

A

L

Porosity

Direct measurement, Vb!

!Regular shape samples (cylindrical , cube etc): Measure the dimensions and calculate the bulk volume!



Porosity

Indirect measurement,, Vb!

!Regular and irregular shape samples!


A

L


Two methods will be discussed:!

!- volume of displaced mercury!

!- gravity method!

Porosity


volume of displaced mercury !

Hg!

Porosity measurement!Vb , bulk volume can be measured directly or indirectly!

Additional volume of mercury after the sample is pushed into the mercury is equal to bulk volume of the sample.!

Balance

Porosity

Gravity!Wtd = weight of dry sample!Wthg = weight of mercury!

Wtb = balance reading due to the sample being immersed into the mercury!

A!

B!

B!A!

Weight of displaced mercury = (Wtb - (Wtd + Wthg))!Volume of

displaced mercury!

= !

Weight of displaced mercury!

Mercury density!

Bulk volume of sample ! =

!Volume of displaced mercury!

Balance

Hg!




H2O!

Porosity

Vp , pore volume can be measured directly or indirectly!

Direct measurement, Vp!

Saturating sample with liquid!Wtd = weight of dry sample!Wtt = weight of saturated sample!

Wtf = weight of liquid in the sample!

!= Wtt - Wtd!Vp = (Wtt - Wtd)/ρf!


H2O!

Porosity


Direct measurement, Vp!

Saturating sample with liquid!


Vb 35 ccWtd 20 gWtt 25 gWf 5 grhof 1.25 g/ccVp 4 cc

Porosity 0.11411.4

Porosity

Indirect measurement, Vp! Grains volume by using Boyle’s

Law!

Helium tank!P1 V1 T1!

P2 V2 T1!

Sample cylinder!

Stainless steel discs! P1 V1 = P2 V2 !

V2 = V1 + Vl + Vs -Vc!

line!



P2 = ?!


Sample cylinder!

Stainless steel discs!

line!

Porosity

P3 V3 T1!

P1 V1 = P3 V3 !

V3 = V1 + Vl + Vs -Vc - Vg!

sample!

Indirect measurement, Vp!

Vp , pore volume can be measured directly or indirectly!Porosity measurement!

Grains volume by using Boyle’s Law! P3 = ?!

Porosity

P1 V1 = P2 V2 ! V2 = V1 + Vl + Vs - Vc!

V1 + Vl + Vs - Vc = V2 = (P1 V1 )/ P2 !

Vg = V1 + Vl + Vs - Vc - V3 !

P1 V1 = P3 V3 ! V3 = V1 + Vl + Vs - Vc - Vg!

Vg = (P1 V1 )/ P2 - (P1 V1 )/ P3 !

Vp = Vb - Vg!

φ = Vp / Vb = (Vb - Vg)/ Vb !

Indirect measurement, Vp!



Grains volume by using Boyle’s Law!

Vl = volume of tubing connecting helium tank to the sample cylinder!

V1 = reference volume of helium tank!

Vs = volume of empty sample cylinder!

Vc = volume of stanless steel discs!

Vg = grains volume of the core sample!


P2 = ?!

P3 = ?!

Vb 35 ccP1 100 psiaV1 100 ccP2 68 psiaP3 85 psia

Vg 29.41 ccVp 5.59

porosity 0.16016.0 %

Vg = (P1 V1 )/ P2 - (P1 V1 )/ P3 !

Porosity

Average porosity!

chapter 2 porosity

Documents